Biometric scanners and in particular finger image scanners are used to identify an individual from their unique characteristics in order to provide secured access to computer systems, secured areas, electronic communications, and electronic financial transactions.
In general finger image identification is accomplished by matching a bid finger image against a reference finger image. This is done by creating a "template" of each image by analyzing the ridges and valleys, and then comparing the two templates to determine if they are similar enough to effect either a successful or failed identification of an individual. In a good quality finger image, the majority of the image surface area consists of ridges that appear as dark lines and valleys that appear as lighter areas.
A significant problem with finger image identification systems is that of false rejection. A false reject happens when a biometric identification system incorrectly refuses access to a user who is actually an authorized user. This happens most frequently because of poor finger image quality obtained from the individual. For example, the finger image obtained by the biometric scanner may be too dark, or too light. First, in a finger image that is characterized as too dark, the valleys appear darker and thicker than normal and it becomes difficult or impossible to distinguish the valleys from the ridges. Several different factors can independently cause a dark finger image. These include a finger that is being pressed too hard or a finger that is too moist. Second, in a finger image characterized as too light, ridges that normally appear as dark areas are lighter in color or appear as broken ridge lines. When a light ridge image is combined with background noise that most scanners produce after repeated usage, it becomes very difficult to distinguish the light ridges from the background noise. Several independent factors can cause a light finger image including a finger that is too dry, a finger that is not being pressed hard enough on the finger image scanner, a finger whose ridges are very fine, and a finger that does not have any ridges at all. An example of each of these symptoms appear as FIGS. 1(a) through 1(d).
In either case, a finger image is that is either too light or too dark is of poor quality and would therefore be difficult to extract a good template using the image. In such cases it is much more difficult to make a proper identification of that individual. This situation often results in the false rejection of an authorized user, making the system appear unreliable.
Conventional scanners often scan images of fingers at pressure levels that are extremely unlikely to produce good images. The scanner's attached processor or logic device thereby wastes time and power determining if a good quality image is present. It is optimal that a scanner only capture an image when it has been determined that the pressure within the acceptable range of T1 and T2 is present and that a good image can be anticipated. This improves response time and saves power since the processor can sleep until a likely good image is present.
When an unsatisfactory image is obtained, it is possible to improve the image by immediately prompting the user to change the way he is using the scanner, instead of simply falsely rejecting the user. In many cases, pressing with less pressure causes the image to lighten, and pressing with more pressure causes the image to darken. Faced with a dark image, a conventional scanner might ask the individual to decrease the pressure on the scanner. Thus, a scanner needs to determine whether or not changing the pressure exerted by the finger of the user would be likely to increase image quality. It is difficult for a conventional scanner to determine this from a simple inspection of the finger image.
It is therefore an object of the invention to provide a finger image scanner that can detect if a problem with a given finger image is related to the exerted pressure, or whether the source of the problem is from another cause yet having the same symptoms.
It is another object of the invention to scan and process a finger image only under conditions which are likely to produce good quality images, thereby improving response time, and decreasing power consumption by not requiring the processor to process images that are almost guaranteed of being poor quality.
It is yet another objective of the invention to alleviate the problem of poor quality images by varying the amount of pressure that is exerted by the finger of the user regardless of other existing causes.
Another objective of the invention is to provide a finger image scanner that uses a minimum number of mechanical or electrical parts, and at a low cost.